Abstract
The nitrogen doping of titanium dioxide nanotubes (TiO2 NTs) was investigated as a result of well-controlled plasma nitriding of TiO2 NTs at a low temperature. This way of nitrogen doping is proposed as an alternative to chemical/electrochemical methods. The plasma nitriding process was performed in a preparation chamber connected to an X-ray photoelectron spectroscopy (XPS) spectrometer, and the nitrogen-doped TiO2 NTs were next investigated in situ by XPS in the same ultrahigh vacuum (UHV) system. The collected high-resolution (HR) XPS spectra of N 1s, Ti 2p, O 1s, C 1s, and valence band (VB) revealed the formation of chemical bonds between titanium, nitrogen, and oxygen atoms as substitutional or interstitial species. Moreover, the results provided a characterization of the electronic states of N–TiO2 NTs generated by various plasma nitriding and annealing treatments. The VB XPS spectrum showed a reduction in the TiO2 band gap of about 0.6 eV for optimal nitriding and heat-treated conditions. The TiO2 NTs annealed at 450 or 650 °C in air (ex situ) and nitrided under UHV conditions were used as reference materials to check the formation of Ti–N bonds in the TiO2 lattice with a well-defined structure (anatase or a mixture of anatase and rutile). Scanning electron microscopy microscopic observations of the received materials were used to evaluate the morphology of the TiO2 NTs after each step of the nitriding and annealing treatments.
Highlights
Article replaced by nitrogen ions. It seems that the new localized N 2p states, which are located above the valence band (VB), narrow the band gap of TiO2 and lead to a shift in optical response toward the visible range.[10,15,17,18]
Such a phenomenon has been noted by Asahi and Morikawa, who showed the presence of nitrogen atoms as substitutional or interstitial dopants[19,20] based on first principle calculations and experimental X-ray photoelectron spectroscopy (XPS) spectra for the N complex species introduced into TiO2
The use of plasma nitriding has proven to be an alternative solution for incorporating nitrogen atoms into the TiO2 NT lattice with respect to chemical/electrochemical methods
Summary
Titanium dioxide (TiO2) is widely used as a photocatalyst because of its high chemical stability (chemical inertness), low cost (earth abundance), and photocatalytic activity under UV light excitation and it is widely used in many applications such as the decomposition of organic pollutants, selective oxidation, hydrogen evolution, solar cells, and others.[1−4] In comparison with other forms of nanostructured TiO2 materials, nanotubes (NTs) are attractive candidates as photocatalytic materials because of their strong light-scattering effects and high surfaceto-volume ratio.[5,6] These highly ordered, vertically oriented tubular structures (with a specific crystal structure) feature a high degree of electron mobility along the tube axis[7] perpendicular to the titanium substrate, which greatly reduces interface recombination as a result of structural distortion at the bottom cap region of the NTs (mixed valence states of Ti: Ti3+ and Ti4+).[8]. Lin and co-authors showed the possibility of using plasma treatment to surface functionalization of various materials including TiO2 for different applications.[16] under optimal conditions of doping processes, oxygen atoms in the TiO2 lattice are Received: January 8, 2020 Accepted: March 26, 2020 Published: April 8, 2020
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